With the staggering prices being paid at auction these days for early mint stamps with never hinged original gum, it can be argued that original stamp gum, by weight, is one of the most valuable substances on earth. Very few examples of stamps issued prior to 1900 have survived with completely undisturbed original gum. Most all the examples that have survived were either very carefully stored outside of stamp albums all these years, or they were part of a larger multiple that was hinged on only 1 or 2 stamps in the multiple.
However, the push by many collectors towards perfection, has caused the demand for never hinged stamps to skyrocket in recent years. Because of this, there have been a very large number of instances in which fakers have added gum to a stamp that lacks it in order to enhance the value. This practice is known in the field as re-gumming, and it can cost a collector dearly if they unwittingly fall victim to it. The reason is simple: re-gummed stamps are typically regarded by the market as being worth the same as a stamp with no gum.
So, how do you avoid being a victim?
The best way of course is to become familiar with what the original gum on a stamp is supposed to look like. However, you have to consider that for many long-running definitive sets, such as the Small Queens or the Admirals of Canada, the appearance of the gum changes over the life of the set. So, in becoming familiar with the types of original gum found on a series of stamps, you will usually need to have examined a representative cross-section of stamps from that set. That can often be costly, but the easiest way to keep the cost down is to study the cheapest value from the set. It will be the best choice anyway, since most of the stamps you find will not be re-gummed because it isn't worth the trouble, and the lowest value in a set usually has the largest number of different printings, and will hence display the greatest variation in the thickness, colour and sheen of the gum. Generally speaking, a re-gummed stamp will never look the same as a stamp with original gum. The reason is that stamp gum is a lot like paint on a wall. If you attempt to match a different brand of paint to an existing brand, you will almost always be able to spot the difference - it may not be colour, per se, but it will differ in either depth, or sheen. So, it is with gum on stamps.
Of course it is impossible to become familiar with the original gum on every single stamp issue from every country in the world, to this degree. So, it is important to understand how the process of gumming stamps works, and what happens to a stamp when it has been re-gummed, so that you know what to look for in spotting a re-gummed stamp.
Gum is usually applied to sheets of stamps after printing, before the stamps are perforated. Some printing is done on pre-gummed paper as well and sometimes, not often, the gumming was done after the sheets were perforated. When the gum has been applied before perforating, there will be very minute areas of damage to the gum that occurs when the stamps are torn along the perforations. The very edge of the perforation tips will lack gum, which has been sheared off of the paper by the tearing motion, and the fine microfibres that make up the paper stock will be exposed, and will be completely free of gum. If you examine the perforations of a stamp with original gum using 10x magnification, you will see this clearly. If the gum has been applied after perforating, you will see the same thing, except you will often see gum inside, or along the edges of the perforation holes, which would not be there on stamps to which the gum has been applied before perforating.
When a stamp is re-gummed, the new gum gets on to the tips of the perforations and into the microfibres, and is almost impossible to remove, without damaging the paper. If you look at a stamp that has been re-gummed very well, with no sloppy gum spots on the face of the stamp and other give-aways, under magnification, you will typically see tiny globules of gum on the perforation tips and microfibres. This will NEVER happen on a stamp with original gum. So, this test is a foolproof one. Some sophisticated re-gummers will attempt to remove this gum by lightly sanding the tips of the perforations, but this will almost always result in very light thinning of the paper where the perforations are, and this can be detected by immersing the stamp in watermark fluid. The thinned areas will appear darker than the rest of the stamp. All stamps will show this thinning at the very tips of the perforations, but if the thinning extends more than 0.5 mm into the perforation tooth, chances are you are dealing with a re-gummed stamp.
A closely related problem to re-gumming concerns re-distribution of original gum to make a stamp appear never hinged when it is not. The common way that this was done was to place the stamp in a humid box called a "sweat box" and allow the gum to become soft and wet. A fine brush would then be used to reapply the gum. However, if it was done poorly, the gum would bleed through the stamps if they were left in the sweat box too long. If you have been collecting for a long time, you will have run into many stamps like this. Even when it is done well, it is next to impossible to disturb the gum and have it dry to the same sheen and appearance as it had when it was first applied. So, if you are familiar with what the original gum is supposed to look like, spotting re-distribution of gum is usually pretty easy. Brushstrokes in gum that are not supposed to have any is usually a sign of re-distribution as well. However, many early issues from the 1870's and before had the gum applied by brush normally, so brushstrokes on these stamps are the norm, and are not indicative of any problem.
So, it does pay to be familiar with the gum for the particular issue you are collecting, but understanding how to check for re-distribution and re-gumming is an important skill to have if you are going to collect mint stamps with original gum issued before 1900.